In recent years, as portable apparatus such as notebook personal computers and cellular phones has been spreading, demand for batteries as power sources thereof has been increasing. In particular, demand for small and lightweight secondary batteries that have high energy density and that can be charged and discharged repeatedly has been increasing.
In order to meet such demand, research and development of secondary batteries have been conducted actively. As portable apparatus has higher functions, the secondary batteries have more energy, and proportionally, amount of heat produced at the time of abnormality increases. When a battery produces heat abnormally, a gas is produced in the battery and internal pressure of the battery increases. Therefore, a safety valve for discharging the gas to the outside of the battery when the internal pressure exceeds a predetermined value is provided in the battery.
Various studies have been made on the safety of battery packs housing such batteries. For example, Patent Literature 1 proposes to cool a high-temperature gas discharged from a battery and discharge the same to the outside of a battery pack. Specifically, it proposes to improve the structure of a case having an exhaust hole. The case is composed of a battery-housing portion, a cylindrical filter portion, and a partition having a vent hole and separating the battery-housing portion and the filter portion. The cylindrical filter portion is composed of a cylindrical fire extinguisher-filled portion and heat insulator-filled portions disposed on both ends of the fire extinguisher-filled portion.